Hydraulic impact tool

Abstract

An impact tool for striking a tool such as a chisel includes a piston mounted in a cylinder. Upper, middle and lower chambers are formed therebetween. Pressure oil is fed into and discharged from the middle and lower chambers to reciprocate the piston in the cylinder under oil pressure. During a downward stroke of the piston, the oil pressure in the lower chamber is kept low. But just before it strikes a tool, the oil pressure in the lower chamber is adapted to increase so as to prevent what is called cavitation or prevent air bubbles mixed in the pressure oil in the lower chamber from growing suddenly owing to a sharp drop in the oil pressure which is caused by the rebound of the piston after striking the tool.

What is claimed is: 
     
       1. A hydraulic impact tool for striking a tool such as a chisel, comprising: a cylinder having the tool slidably mounted therein at lower end thereof;   a piston reciprocably mounted in said cylinder for striking the tool during its downward movement, wherein said piston having an upper small-diameter portion and a lower small-diameter portion and upper and lower large-diameter portions between said upper and lower small-diameter portions, and a middle small-diameter portion between said upper and lower large-diameter portions, wherein said cylinder having an upper chamber filled with a gas to apply gas pressure to the top of said piston, and a middle chamber and a lower chamber defined directly above said upper large-diameter portion and directly below said lower large-diameter portion, respectively;   a valve chest connected to said middle chamber and said lower chamber and an oil supply port and an oil discharge port;   a valve body slidably mounted in said valve chest; and   an oil circuit having an oil passage communicating said middle small-diameter portion with said valve chest for controlling the communication between said middle chamber and said lower chamber on one hand and said oil supply port and said oil discharge port on the other hand to alternately raise and lower said piston under the pressure of said gas and oil,   and wherein said oil circuit is so arranged that prior to said piston striking the tool, the pressure oil will be fed into said lower chamber to increase the oil pressure in said lower chamber, and wherein said middle small-diameter portion communicates with said valve chest through said oil passage to raise and lower said valve body in said valve chest.   
     
     
       2. A hydraulic impact tool as claimed in claim 1, wherein said upper small-diameter portion has a smaller diameter than said lower small-diameter portion, in order to lower said piston both by the pressure of the gas in said upper chamber and by the difference between the oil pressure applied to the top of said upper large-diameter portion and the oil pressure applied to the bottom of said lower large-diameter portion.